Manufacture of monolithic LED arrays for electroluminescent display devices

ABSTRACT

In the manufacture of a monolithic LED array, in particular a matrix array, in which portions of the array are electrically isolated from one another by channels cut or etched through the slice of n-type material, the channels are formed in two stages, relatively wide channels first being formed from the back surface of the slice to within 50 microns of the front surface, and filled with a glass frit which is bonded to the semiconductor material on each side of the channel, then narrower channels are formed from the front surface of the slice to meet the glass in the initial channels, and are similarly filled with glass frit. A glass powder suspension is introduced into each set of channels by spreading or spinning over the surface of the slice, or by electrophoretic deposition and/or capillary action, then the liquid suspension medium is removed and the glass powder is sintered.

This invention relates to the manufacture of monolithic arrays oflight-emitting semiconductor diodes (hereinafter referred to as "LEDs")for electroluminescent display devices of the type including such anarray and means for applying operating voltages to the diodes.

A monolithic LED array can be manufactured by a process (hereinafterreferred to as a process of the type specified) which includes the stepsof providing a slice of n-type semiconductor material, forming aplurality of discrete regions of p-type semiconductor material in thesaid slice by diffusing p-type material or a p-type dopant into onesurface of the slice, to delineate the individual diode areas, formingmetal contacts on the surfaces of the p-type and n-type regions of theslice and connecting groups of the p-type contacts together as requiredfor addressing the array, at a convenient stage in the process formingone or more isolation channels through the n-type region of the slicefor electrically isolating portions of the array from one another asrequired, and mounting the whole structure on a suitable insulatingsubstrate. That surface of the semiconductor slice which carries thep-type regions, and the contacts formed on said regions, willhereinafter be referred to as the "front" of the slice and the "front"contacts respectively, and the opposite surface of the slice on whichthe semiconductor material is wholly n-type and any contacts formed onthis surface will be referred to respectively as the "back" of the sliceand "back" contacts. The substrate may be bonded either to the back ofthe slice or to the back contacts.

For example, in the manufacture of a matrix array consisting of aplurality of LEDs arranged in rows and regularly spaced so as to formcolumns orthogonal to the rows, the diodes in each column having acommon back contact on the n-type surface, and the diodes in each rowhaving individual front contacts on the p-type areas which are connectedtogether by a continuous conductor (usually known, and hereinafterreferred to, as a "beam lead") for addressing the row in operation ofthe device, the required electrical isolation between the columns inrespect of the n-type regions and common contacts may be effected bycutting or etching channels from the back of the structure, through thecommon contact layer and the n-type slice, in such a manner that, whilethe columns are completely isolated from one another, the overlaid beamleads are undamaged. This method of isolation requires the array to besubsequently handled, for the purpose of mounting the array on to asubstrate, in a highly vulnerable and fragile state, as the array isheld together solely by the adhesion of small areas of the beam leads tosmall areas of the semiconductor strips constituting the diode columns.

In another method of effecting the isolation, the array is first mountedon its substrate, and then the requisite channels are etched from thefront of the slice, after the beam leads are formed, using theundercutting properties of the etch to provide isolation. However, thistechnique is limited to coarse geometry arrays, due to the anisotropy ofsuch etches.

It is an object of the present invention to provide an improved processfor the manufacture of a monolithic LED array, whereby handling of thestructure subsequently to the formation of the isolation channels isfacilitated, and a large area, beam leaded, high resolution array can beobtained.

According to the invention, in a process of the type specified for themanufacture of a monolithic array of light-emitting semiconductordiodes, for an electroluminescent display device, each said isolationchannel is formed in two stages, the said process including the furthersteps of mounting the front surface of the slice of n-type material on afirst support layer, forming one or more initial channels from the backsurface of the slice through a major part of the thickness thereof,filling each said channel with a suspension of glass powder particles ina liquid medium, removing the liquid from the channel or channels sofilled, heating the slice to a sufficiently high temperature to causethe glass particles in each channel to sinter so as to form a glass fritcontained in the channel and bonded to the semiconductor material oneach side of the channel, and repeating the said steps of filling thechannel or channels with glass powder suspension, removing the liquidand sintering the glass, successively, for the number of times requiredto fill each channel completely with said glass frit, then mounting theback surface of the slice on a second support layer and removing saidfirst support layer from the front surface of the slice, then formingone or more further channels from the front surface of the slice, suchthat a said further channel is formed to correspond in position to eachsaid initial channel, each said further channel being narrower than thecorresponding initial channel and extending through the slice for asufficient depth to meet the glass frit filling the said initialchannel, and filling said further channel or channels with glass frit bycarrying out said steps of filling each channel with glass powdersuspension, removing the liquid, and sintering the glass, to form acomposite glass-filled channel or channels each consisting of a narrowfront portion and a wider back portion.

Each said initial channel is suitably terminated at a distance of lessthan 50 microns from the front surface of the n-type slice.

After the said further channel or channels have been filled with glassfrit, the second support layer may be removed from the back surface ofthe slice to permit further processing, for example if it is desired toform back contacts on the n-type regions of the slice or to mount theslice on a substrate of different composition from the support layer.Alternatively, the second support layer may be retained to constitute,or to be mounted on, the substrate, if back contacts are not required.

The isolation channel or channels may be formed by mechanical cutting orby chemical etching. Usually a plurality of said composite isolationchannels will be required in an LED array, and the invention willhereinafter be described with reference to such an array, but it is tobe understood that the operations described are equally applicable tothe manufacture of an array having a single composite isolation channel.The glass employed for filling the channels may be of any suitablecomposition that can be sintered at a temperature which is not so highas to cause damage to the semiconductor material; the glass must alsohave a thermal expansion coefficient compatible with that of thesemiconductor material, so that no strains or cracks will be produced ineither material during the sintering process.

Usually the p-type material or dopant is diffused into the requiredareas of the front surface of the slice after the composite isolationchannels have been formed and filled with glass frit. However, in somecases, if the temperature to which the slice is required to be heatedfor effecting the diffusion of the p-type material into the n-typematerial is appreciably higher than the sintering temperature of theglass employed for filling the channels, it may be desirable to form thediffused p-type regions before the formation and filling of thechannels.

The filling of the isolation channels with a glass frit bonded to thesemiconductor material is advantageous in that it produces a compositeslice of robust construction, enabling the subsequent processing stepsto be carried out readily without risk of damaging the semiconductorslice; the planar composite glass/semiconductor structure of the frontand back surfaces of the slice allows diffusion of the p-type materialand formation and connection of the front contacts, and formation ofback contacts if required, to be carried out by the usual techniques.Furthermore, the glass-filled channels improve the mechanical strengthof the completed array while maintaining the required electricalisolation between n-type regions. The formation of the channels in twostages, in accordance with the invention, is advantageous in that thefilling of the channels with a glass powder suspension is facilitated,in the case of the initial channels by their greater width, and in thecase of the further, narrower, channels by their small depth.Furthermore, in operation of an LED array having glass-filled isolationchannels formed by this two-stage process, the degree of resolutionobtainable will be determined by the width of the narrower, frontportions of the channels, and can thus be higher than that obtainablewith conventional isolation channels of uniform, somewhat greater, widththroughout the slice.

The liquid suspension of glass particles employed successively forfilling the initial and further isolation channels may consist of aslurry of glass powder in a binder medium which can be applied, forexample by spreading or spinning, over that surface of the slice inwhich the openings of the channels are situated, so that the slurryfills the channels. The excess slurry is removed from the surface areasof the slice between and outside the channels before removal of theliquid medium from the channels and sintering of the glass therein. Thebinder medium may consist of a photoresist, or any suitable knownbinder, in an organic solvent, and after evaporation of the solvent thebinder may be removed by appropriate chemical means, or may be oxidizedand vaporised by heating in the presence of oxygen, or in the case of aphotoresist may be exposed to ultra violet radiation and then removed byimmersion of the slice in a standard developer solution. Duringsintering the glass flows and is thus reduced in volume: it is thereforenecessary to repeat the procedure of filling the channels with slurry,removing solvent and binder, and sintering the glass several times inorder to effect complete filling of the channels.

An alternative method of introducing glass particles into the isolationchannels is by electrophoretic deposition: the semiconductor slice, withthe required channels formed therein and the remaining surface areas ofthe slice suitably masked, is immersed in a suspension of glassparticles in a suitable liquid containing a surfactant which imparts anelectric charge to the glass particles, an electrode is also immersed inthe suspension, the semiconductor slice functioning as the secondelectrode, and an electric field of appropriate polarity is appliedbetween the slice and the electrode to cause the glass particles to beattracted to the slice and deposited in the channels. The slice is thendried in an oven to remove the liquid introduced into the channels withthe glass particles, and the slice is heated to sinter the glass.

In another alternative method of depositing glass particles in thechannels, the slice is immersed in a suspension of glass particles in asuitable liquid having high surface tension or containing a surfactant,the slice being so oriented in the liquid that the longitudinal axes ofthe channels lie substantially orthogonally to the surface of theliquid, then the slice is slowly withdrawn from the liquid: duringwithdrawal of the slice, the glass suspension is drawn into the portionsof the channels rising above the surface of the liquid, by capillaryaction.

One preferred method of depositing the glass particles in the channelscomprises a combination of the electrophoretic and capillary actionprocedures described above: thus the slice is immersed in a glass powdersuspension as aforesaid, with the longitudinal axes of the channelsoriented orthogonally to the surface of the liquid, and electrophoreticdeposition of glass particles in the channels is effected first, andthen the slice is slowly withdrawn from the suspension to promotefurther deposition by capillary action. This procedure is advantageous,since the electrophoretic action ensures coverage of the innermostregions of the channel walls with glass particles, and the subsequentcapillary action ensures that the channels are completely filled withglass particles.

Suitable semiconductor materials for use in the manufacture of the LEDarray include, for example, gallium arsenophosphide, gallium arsenide,gallium indium phosphide, gallium aluminum arsenide, gallium phosphide,and gallium indium arsenide phosphide, in each case containing suitabledopants for producing n-type or p-type material as required: a preferredp-type material is zinc doped gallium arsenophosphide, and preferredn-type materials are gallium arsenophosphide doped with selenium ortellurium, and silicon doped gallium arsenide. The slice of n-typesemiconductor material may be of graded composition, and is suitablyprepared by epitaxial deposition of material of the required compositionor range of compositions on an initial thin slice of similar or alliedcomposition. The diffusion of p-type material into the front surface ofthe slice is carried out by a well-known process, the pattern of p-typeregions being delineated by conventional photolithographic techniques.

The contacts are composed of suitable metals, for example chromium,nickel, nickel-chromium alloy, or titanium, overlaid with gold, and maybe formed by vacuum evaporation in known manner, the required contactpatterns being delineated by photolithographic techniques, and thecontacts so formed may be thickened by electroplating if desired. Therequired connections between front contacts on the p-type regionspreferably consist of beam leads, formed by depositing, suitably byelectroplating, photolithographically delineated strips of metal, forexample gold, on the appropriate portions of the front of the slice soas to overlay groups of the p-contacts. Alternatively the connetionsbetween the p-contacts may be made by stitch-bonding.

The contacts for the n-type regions of the array may be in theconventional form of a continuous metal layer covering the whole of theback surface of each such region. However, for convenience inmanufacture of the device, the n-contacts may advantageously be locatedon the front surface of the n-type slice, so that all contacts areaccessible from the front of the array; a single small contact on thefront of each n-type region, for example located at one end of eachcolumn in a matrix array, may be sufficient, but preferably twon-contacts are provided on each region, for example one at each end ofeach column, to ensure reliability of operation of the device. Theadvantage of such an arrangement is that all the contacts, on bothp-type and n-type regions, can be connected to conducting tracks on thesubstrate by stitch-bonding from the contacts on the edges of the frontof the slice to the said tracks. If desired, where n-contacts arelocated on the front of the slice, additional continuous contacts may beprovided over the back surfaces of the n-type regions, to ensure thatelectric current introduced through the front n-contacts is evenlydistributed throughout the n-type regions.

Suitable materials for the substrate of the array include, for example,ceramic alumina, sapphire, and high resistivity silicon. If the LEDarray includes back contacts, an area of the substrate corresponding tothe area of the array is metallised to enable the contacts to be bondedto the substrate, but if no back contacts are present on the array, theback surface of the array, composed of n-type semiconductor material andthe glass frit in the isolation channels, is bonded directly to theinsulating substrate material. The substrate also carries the requisiteconducting tracks to form leads for addressing the array in operation ofthe device.

Some specific processes, in accordance with the invention, for themanufacture of high resolution matrix LED arrays, will now be describedby way of example, with reference to the accompanying diagrammaticdrawing, which is a perspective view of a portion of an array havingisolation channels formed by the two-stage process of the invention.

EXAMPLE 1

The array shown in part in the drawings consists of a plurality ofcolumns formed from a rectangular slice of n-type semiconductor material1, composed of silicon doped gallium arsenide (GaAs) 1a overlaid bytellurium doped gallium arseno-phosphide (GaAsP) 1b, with an ohmiccontact 2 covering the whole of the back surface of each column andbonded to a ceramic substrate 3, and with front surface diffused regionsof p-type GaAsP, 4, delineating the diode matrix and provided withindividual front ohmic contacts connected together in rows approximatelyorthogonal to the columns. The columns are separated from one another byisolation channels filled with sintered glass frit, indicated bystippling, each channel consisting of a relatively wide portion 5a,extending from the back surface of the n-type semiconductor slice 1 to adistance of 25 microns from the front surface of the slice, and anarrower portion 5b formed from the front surface of the slice to thecentre of the portion 5a. The portions 5a and 5b are, for example, 100microns and 25 microns wide respectively. Each of the front contactsconsists of a rectangular metal pad 6 overlapping the p-type region,with a narrow conducting strip 7 formed around the edge of the remainderof the p-type region; these contacts are connected together bycontinuous beam leads 8 laid across the array, overlying the respectiverows of contact pads 6 and the intervening glass-filled channels 5b: oneof the beam leads has been omitted from the drawing, in order to showone row of contact pads 6. Alternatively, the rows of front contact pads6 may be connected together by stitch-bonding (not shown in thedrawing).

In the manufacture of the array described above with reference to thedrawing, an n-type slice 1 of the desired thickness is produced bydepositing an epitaxial layer of tellurium doped GaAsP on an initialslice of silicon doped GaAs, and a supporting layer of silica or siliconnitride is deposited over the front (GaAsP) surface of the slice. Thechannels 5a are then cut or etched from the back surface of the slice,and are filled with a proprietary glass powder slurry in a photoresistbinder medium, by spreading or spinning the slurry over the frontsurface of the slice, the excess slurry being scraped off thesemiconductor surface areas between and outside the channels. The sliceis then heated to evaporate the solvent of the slurry, further heated inoxygen or air to fire off the binder, and finally heated at 750° C. tosinter the glass particles in the channels to form a glass frit bondedto the semiconductor columns. The slice is then re-mounted on a supportlayer applied to its back surface, the support layer on the frontsurface is removed, for example by lapping or polishing, and thechannels 5b are cut or etched from the front surface and are filled withglass frit by the same procedure as that employed for filling thechannels 5a with frit. The support layer is then removed from the backsurface of the slice.

The required p-type regions are then formed by diffusing zinc into thesurface areas 4, the diode matrix pattern being delineated in a suitablediffusion mask material by conventional photolithographic techniques andthe slice being heated to 700° C. for the diffusion process.

The front contacts 6, 7 are formed by vacuum evaporation of chromium andthen gold, photolithographic techniques again being used for delineatingthe contact pattern, and the back contacts 2, composed of a gold-tinalloy, are deposited on the semiconductor columns by a procedure similarto that used for forming the front contacts. The array is heated to 500°C. to effect partial diffusion of the metal of the front and backcontacts into the semiconductor material, and then the beam leads 8 areformed by electroplating gold on to the requisite areas of the frontsurface of the array through a photolithographically produced mask.Finally the array is mounted on the substrate 3 by soldering the backcontacts to a delineated metalised surface area of the substrate. Thesubstrate suitably consists of either high resistivity silicon with asurface layer of thermal or pyrolytic silicon oxide overlaid by gold, orceramic alumina metallised with gold, and the solder used may be agold-germanium alloy; the glass frit strips on the back surface of thearray are not wetted by the solder. The beam leads 8 are connected toconducting tracks on the substrate (not shown) by stitch-bonding.

EXAMPLE 2

The process of this example includes an alternative method of fillingthe isolation channels with glass powder suspension, which can beemployed in the manufacture of an array of the form shown in thedrawing.

The n-type semiconductor slice is prepared as described in Example 1,and both the front and back surfaces of the slice are coated withsilicon nitride (or other suitable non-conducting masking material). Therequired isolation channels 5a are delineated by removing portions ofthe silicon nitride coating from the back surface, by aphotolithographic method or by cutting through the coating, and thechannels are cut or etched in the n-type material. The slice is thenimmersed in a suspension of glass particles in isopropyl alcoholcontaining 0.25% to 1.0% by volume of a surfactant, suitably acommercially available material designated "FC-807", the proportion ofglass particles in the liquid being 10% weight/volume, and the slicebeing so oriented in the suspension that the longitudinal axes of thechannels are orthogonal to the surface of the liquid. A platinumelectrode is also immersed in the suspension, and a current of 6 to 8 mAis passed through the suspension between the slice as anode and theelectrode as cathode, for 4 to 12 hours: the glass particles arenegatively charged by the surfactant and are therefore deposited in thechannels in the semiconductor slice. After switching off the current,the slice is slowly withdrawn from the liquid, so that as the channelsemerge above the surface of the liquid, they are completely filled withglass suspension drawn into them by capillary action. The slice is thendried in an oven at 100° C., and heated to effect sintering of the glassparticles.

The above-described procedure is repeated for delineating and formingthe channels 5b from the front surface of the slice, and filling themwith glass powder suspension and converting the latter to glass frit.

After completion of the formation of the glass-filled channels, thediode areas are delineated by removing further portions of the siliconnitride coating from the front surface of the slice, the silicon nitrideis removed from the back of the slice, and the p-type regions, contactsand beam leads are formed as described in Example 1, the completed arrayfinally being mounted on a substrate.

It will be understood that, in either of the methods described in theabove specific examples, the steps of filling each portion of theisolation channels with glass powder slurry or suspension, removing theliquid medium and sintering the glass may be repeated as necessary toeffect complete filling of the channels with glass frit, before thefurther processing steps are carried out.

We claim:
 1. A process for the manufacture of a monolithic array oflight-emitting semiconductor diodes, for an electroluminescent displaydevice, which includes the steps of providing a slice of n-typesemiconductor material, forming a plurality of discrete regions ofp-type semiconductor material in the said slice by diffusing p-typematerial into the front surface of the slice, to delineate individualdiode areas, forming individual metal contacts on the surfaces of saidp-type regions and connecting said contacts together in groups asrequired for addressing the array, forming at least one isolationchannel through the n-type slice for electrically isolating portions ofthe array from one another, forming at least one metal contact on asurface of the n-type region of each of said mutually isolated portions,and bonding the back surface of the structure so formed to an insulatingsubstrate, wherein each said isolation channel is formed in two stages,the said process including the further steps of(a) mounting the frontsurface of the slice of n-type materal on a first support layer, (b)forming at least one initial channel from the back surface of the slicethrough a major part of the thickness thereof, (c) filling each saidchannel with a suspension of glass powder particles in a liquid medium,(d) removing the liquid from each channel so filled, (e) heating theslice to a sufficiently high temperature to cause the glass particles ineach channel to sinter so as to form a glass frit contained in thechannel and bonded to the semiconductor material on each side of thechannel, (f) repeating said steps (c), (d) and (e) successively for thenumber of times required to fill each channel completely with said glassfrit, (g) mounting the back surface of the slice on a second supportlayer, (h) removing said first support layer from the front surface ofthe slice, (i) forming at least one further channel from the frontsurface of the slice, such that a said further channel is formed tocorrespond in position to each said initial channel and extendingthrough the slice for a sufficient depth to meet the glass frit fillingthe said initial channel, (j) and carrying out said steps (c), (d), (e)and (f) to fill each said further channel with said glass frit, to format least one composite glass-filled channel consisting of a narrow frontportion and a wider back portion, the formation of each of said initialand further channels and filling thereof with said glass frit beingcaried out before the formation of the said contacts on the p-type andn-type regions of the slice.
 2. A process according to claim 1, whereineach said initial channel is terminated at a distance of less than 50microns from the front surface of the n-type slice.
 3. A processaccording to claim 1, wherein each said initial channel is formed to awidth of 100 microns, and each aid further channel is formed to a widthof 25 microns.
 4. A process according to claim 1, wherein subsequentlyto the filling of each said further channel the said second supportlayer is removed to permit further processing of the back of the slice.5. A process according to claim 1, wherein the said second support layeris retained on the back surface of the slice to constitute at least partof the said substrate.
 6. A process according to claim 1, wherein thesaid suspension employed successively for filling the said initial andfurther isolation channels consists of a slurry of glass powder in abinder medium, and wherein the said slurry is applied over that surfaceof the slice in which each channel opening is situated, so as to filleach channel, the excess slurry being removed from the surface areas ofthe slice before removal of the binder medium from each channel andsintering of the glass therein.
 7. A process according to claim 6,wherein the said binder medium consists of a photoresist in an organicsolvent, and wherein the binder is removed by exposing the slice toultra violet radiation and then immersing it in a developer solution. 8.A process according to claim 1, wherein for introducing glass particlesinto the said initial and further isolation channels formed in the sliceof n-type semiconductor material, the remaining surface areas of theslice are masked, the slice is immersed, together with an electrode, ina suspension of glass particles in a liquid containing a surfactantcapable of imparting an electric charge to the glass particles, and anelectric field of appropriate polarity is applied between the slice andthe electrode to cause electrophoretic deposition of glass particls ineach channel.
 9. A process according to claim 1, wherein for introducingglass particles into the said initial and further isolation channelsformed in the slice of n-type semiconductor material, the slice isimmersed in a suspension of glass particles in a liquid having highsurface tension, the slice being so oriented in the suspension that thelongitudinal axis of each channel is disposed substantially orthogonallyto the surface of the liquid, and the slice is slowly withdrawn from thesuspension, so that the suspension is drawn into each channel, risingabove the surface of the liquid, by capillary action.
 10. A processaccording to claim 1, wherein for introducing glass particles into thesaid initial and further isolation channels formed in the slice ofn-type semiconductor material, the remaining surface areas of the sliceare masked, the slice is immersed, together with an electrode, in asuspension of glass particles in a liquid containing a surfactantcapable of imparting an electric charge to the glass particles, theslice being so oriented in the suspension that the longitudinal axis ofeach channel is disposed substantially orthogonally to the surface ofthe liquid, an electric field of appropriate polarity is applied betweenthe slice and the electrode to cause electrophoretic deposition of glassparticles in each channel, then the electric field is removed and theslice is slowly withdrawn from the suspension, so that furtherdeposition of the suspension in each channel rising above the surface ofthe liquid is effected by capillary action.